1. Field of the Invention
The present invention relates to an electric press and a method for calibrating the electric press.
2. Description of the Related Art
An electric press performs machining, such as indentation, compression or the like, on a workpiece by driving a ram by using an electric motor as a power source. A strain column to which a load cell is adhered is mounted on the front end of the ram. The electric press detects a load value applied to the workpiece from the output value of the load cell, and controls the driving of the ram while comparing the detected load value with a desired load value to be applied to the workpiece.
The electric press includes an instrumentation amplifier and an analog-to-digital (A/D) converter. Furthermore, the electric press includes a conversion table between amplified values and load values that is used in the control of the ram. That is, the electric press detects the corresponding load value by amplifying the output of the load cell, converting the output into a digital value and then converting the digital value into the load value. Generally, the reason why the electric press includes the instrumentation amplifier is that an analog signal output by load cell is weak. Furthermore, the reason why the electric press includes the conversion table is that amplified values are not consistent with load values due to nonlinearity among gain and offset values set for the instrumentation amplifier, the output values of the load cell, and actual load values. In the following description, a load actually applied to a compression target, such as a workpiece or the like, is referred to as an “actual load,” and a value used in the control of the ram through conversion by the load cell, the instrumentation amplifier, the A/D converter, or the conversion table is referred to as a “load value.”
When the electric press is installed in a factory, a calibration operation is performed. During the calibration operation, a reference load cell is used as a compression target in placed of the workpiece W, and output values on an electric press side are compared with the output values of the reference load cell for various loads while the loads are being applied to the reference load cell. Furthermore, the gain value, offset value and conversion table of the instrumentation amplifier are adjusted.
FIG. 19 is a graph plotting changes in the output value of the load cell over time under the same load. A correlation between the output value of the load cell and the load changes over time. The hardening of a bonding agent used to adhere the load cell to the strain column is viewed as one reason for the changes. Accordingly, it is desirable that the calibration operation of the electric press is periodically performed after the installation of the electric press. During the calibration operation, it is recommended that the reference load cell to which loads are applied is loaded on a die set spring. The die set spring functions to absorb a load, and can prevent an excessive load from spreading to the electric press and a facility, on which the electric press is mounted, even when the ram overshoots.
However, once the electric press has been installed in a factory, there occurs a case where it is difficult to install a die set spring due to a difference in the size of workpiece and the stroke of the die set spring. When the ram overshoots excessively in without a die set spring, a load must be absorbed by the stiffness of the electric press or facility. When the stiffness of the electric press or facility is represented by a spring coefficient, the spring coefficient is considerably greater than that of the die set spring. Accordingly, the great overshoot of the ram that occurs due to the absence of the die set spring may cause damage to the electric press and the facility.
Conventionally, when it is difficult to install a die set spring during a calibration operation performed to deal with a change in the load cell over time, the ram must be moved at low speed in order to reduce an overshoot to a considerably small value. The reason for this is that the quantity of an overshoot is proportional to the speed of the ram. Alternatively, the calibration operation must be performed after the electric press or instrumentation amplifier has been removed from the facility.
The calibration operation requiring the low speed movement of the ram and the calibration operation requiring the removal of the electric press or instrumentation amplifier take excessively long periods of time. During the calibration operation, the operation of the facility is stopped. Accordingly, when the calibration operation takes an excessively long period of time, a reduction in the manufacturing efficiency of workpiece becomes serious.
It may be contemplated that measures are taken to suppress a reduction in manufacturing efficiency by somewhat increasing the intervals at which the calibration operation is performed. However, when the output value of the instrumentation amplifier deviates from the A/D convertible range of the A/D converter due to a change in the load cell over time, the output value of the A/D converter is saturated and thus an appropriate load cannot be applied to a workpiece. Furthermore, even when an excessive load above a rated load is applied to the electric press, it is difficult to identify the excessive load, resulting in damage to the electric press.
To prevent the clipping of the A/D converter, the multiplication factor of the instrumentation amplifier must be set to a small value, i.e., a gain value must be set to a small value, in order to prepare for a change in the load cell over time. As a result, the resolution of the load value is degraded, and thus the electric press cannot offer its inherent performance.